Bike problems...

[Phatty5BMW]

Hey guys/gals, I have a question for ya......

Does anyone know anything about old hondas? I got a 81 CB750c I need guidence on and the shop won't do squat for less than 900$ lol. It's mostly carb issues. If anyone could help I would be greatly appreciated

Thanks,
Pat

[Barney Paull-Edwards]

If they are Mikuni`s, take them to bits, clean old petrol out, build carefully and balance with 4 pipe Davia balancer, remember not to lock up connectors until balanced! 9 times out of 10 problems are dirt or old petrol due to lack of use.

[Barney Paull-Edwards]

Oh, I forgot, then attach a chain to frame above engine, throw into lake and voila, one good mooring.

[Phatty5BMW]

Oh, I forgot, then attach a chain to frame above engine, throw into lake and voila, one good mooring.

Naw they are the kei-hin or something. I'm kinda pissed now, I bought another set of carbs off the bay, they have different part numbers... The ending digits on my original end it SI the others end in RE...

I couldnt check what mine were because the bike was being held hostage for more than its worth lol.. "if i only had a float...."

-Pat

[DaveVoorhis]

Whether Keihin or Mikuni carbs, the process is the same: Remove carbs, disassemble completely (keeping each carb's bits separate from the others), throw away all the rubber bits, clean thoroughly with strong carb cleaner until they're shiny inside like new, replace the jets because they'll be clogged with gunge, install all new rubber bits and gaskets, then reassemble and adjust the float levels according to the shop manual or (as a last restort) a Chiltons or Haynes manual. And replace the intake boots -- they're probably cracked, and de-rust the fuel tank and replace the filter screen. Check the compression, too -- you may have sticking or burned valves.

[DaveVoorhis]

Ah yes: Synchronise the carbs when you're done using a set of Carb Stix.

[Phatty5BMW]

Well original carbs has a cracked float. Float levels are non-adjustable. Not sure about taking float off new carbs and putting on old.

Problem with old, wouldnt rev past 4k, took apart cleaned, sat 4 years....putting those carbs on is a bitch..... 1 intake boot is cracked, but that shouldnt cause the bike to not start at all.

New carbs were off of a running bike. Carb 3 is a little different as its missing what looks like to be a choke needle or something but it has a nipple on the engine side of the carb. The original carbs have this.

What I am not sure of is whether the float itself is interchangeable with the original carbs being the float level is non-adjustable.

From all the information I can find, the years of production of the bike 79-82 from what I understand is that they are all teh same carbs.

The fuel distributor that feeds from the tank, has that extra tube on it, as the new carbs installed has no where to hook too. I don't know wtf to do...

Sorry for bugging you about this but its bugging the hell outta me and i know you guys are very resourceful

Thanks
-pat

[DaveVoorhis]

It's been over twenty years since I worked on Honda in-line fours, and almost ten years since I've owned one, so I'm really stretching my dodgy memory here -- you might be better off checking on a Honda forum. Anyway... As I recall, Carb 3 is the reference carb for synching the carbs, so it won't have an adjuster. The others are adjusted to match it. The extra tube on the tank fixture should be for the vacuum-operated petcock valve -- there should be a nipple on the intake (possibly on Carb 3???) where it connects. If the bike's not starting, it could be because there's no vacuum to operate the petcock.

To check for float interchangeability, simply eyeball it: If it looks identical, it is. Are you sure they're not adjustable? There's usually a tang that can be bent to adjust them.

If it's not starting at all, check the usual suspects: Verify you've got spark and pull a plug to verify fuel delivery to the cylinder(s). Old, nasty carbs and a rusty tank can be so plugged up that no fuel gets through, but I'd suspect the petcock first.

[whiskychaser]

Well original carbs has a cracked float. Float levels are non-adjustable. Not sure about taking float off new carbs and putting on old.

Problem with old, wouldnt rev past 4k, took apart cleaned, sat 4 years....putting those carbs on is a bitch..... 1 intake boot is cracked, but that shouldnt cause the bike to not start at all.


Thanks
-pat

So your problem is 1. You got a cracked float 2. Wont rev over 4k 3. Wont start now? Got a couple of mates run a bike shop for years. FFS if you only need a float and a bit of info no promises but will see what I can do

[Phatty5BMW]

Ok, Well pulled the carbs apart. Same float. I changed the floats over, made sure all my floats work correctly. Carb #2 is the primary carb. I checked all the bowls, carb 2 seems to have the most fuel and carb 4 has the least. I am going to pull them off again and blow everything out again. I am going to clean the plugs, they get spark but hey its a possiblitly.

The one thing i am not sure of is that automatic fuel cock. Its got 2 main gas and 2 vaccum. Kind of stumped, i know the one obviously goes to the tank and the carbs, but the vacuum. air only blows one way. so would the air flow towards the plug on carb 2 or the main rail in between the carbs.

If it wasn't 4 years since i took this apart, I could remember.

Since i have been cranking this thing over and over, out of all 4 exhaust tubes, seems as if cyl 2 is getting hot and not the others. I primed cyl 4 and it fired for a split second and exhaust came out so i know there is spark atleast.

I think it is defenitly a fuel issue, I am kind of leaning towards the fuel rail clogged or this fuel diaphram just isnt working (or i hooked it up wrong)

I have tried honda forums too by the way. When looking for forums you just can't find a place like this anywhere... not even on google.. all others is non-useful junk.. This has to be the best board on the net for sure.

Thanks for the help guys!!!!!
-Pat